Three examples of reversible temperature indicators include liquid crystal indicators, thermochromic inks, and diacetylinic polymers. One problem with these temperature indicators includes limitations on the transition temperature for the indicator, in particular for thermochromic inks and diacetylinic polymers. Thermochromic inks and diacetylinic polymers require about 5-10° C. for the color change to occur, which severely limits their accuracy for temperature indication.
Another problem with some of these known reversible temperature indicators is the color transition, in particular with the gradual change over the broad temperature range mentioned above for the thermochromic inks or to more than one color over a range. Some liquid crystals change from red to green to blue over a temperature range, which can lead to confusion when determining the exact temperature. Moreover, liquid crystals are transparent below the threshold temperature range and also above, which could lead to a false reading.
Thermochromic inks, which typically include a leuco dye, a developer, and a melt point control chemical (usually a fatty acid ester and/or alcohol), are more complex than the formulations disclosed herein. Moreover, some thermochromic inks require the leuco dye and the developer to be kept separate from one another until activation is desired. The need to keep the activator separate from a dye, for example, adds complexity to the overall structure of the temperature indicator and increased difficulty in manufacturing. Thermochromic inks are also susceptible to fading if exposed to UV light.
Another problem with these known reversible temperature indicators is the expense of the materials and limited suppliers. Diacetylinic polymers as raw materials are very expensive and are susceptible to fading and shifting of the color transition temperature if exposed to ultraviolet light, electron beam, or gamma radiation. Liquid crystals and thermochromic inks have limited suppliers and are moderately to highly expensive.
Accordingly, there is a need for a more economic reversible temperature indicator that solves these problems.